If you encounter garbled characters when entering Japanese text in the Sticky Note of Houdini Indie (Steam version) with Houdini version 19.5.640 on Windows 10, try installing Japanese fonts, ensuring Unicode support, using the Japanese IME for input, checking localization settings, updating the software and OS, and running Houdini in compatibility mode for a Japanese environment. If the issue persists, seek assistance from the Houdini support or Steam community for further guidance.
The problem here is a regression bug that was introduced in Houdini 19.5.639 and fixed by 646, which also fixed a number of other long standing multi-byte string problems along the way, so it has nothing to do with your OS, font, or Unicode support, so it's best to avoid 19.5.640 entirely if you are relying on this type of workflow. The newest production build is 19.5.682 now.